The invention relates to refrigeration, and more particularly to a vapor-compression refrigeration system utilizing a liquid-gas coolant with a continuously operating compressor, and with flow of coolant through a valve leading to the evaporator proportioned in response to sensed temperature of a fluid body being cooled at the evaporator. Extended load conditions and temperature ranges are provided by one or more bypass loops controlled by proportional flow valves.
In commercial refrigeration systems, cycling of the compressor causes cycling of refrigerant valves between open and closed positions; and this cycling leads directly to a great number of failures. In systems that use hot gas bypass, often called non-cyclic systems, referring to the fact that the system compressor does not shut off, there is a repeated cycling of valves between on and off positions. Typically, these are diaphragm valves, either thermostatic expansion valves (TEV) or automatic expansion valves (AEV), or solenoid valves. Due to metal fatigue failures start occurring at around 100,000 operations of diaphragm (metal bellows) valves, which can often occur in less than one year of operation for many compression chiller systems.
Other cyclic devices also fail. Solenoid valves fail to open or close after repeated cycling. Solenoid coils fail, as do coil drivers in temperature controllers. Home refrigerators, on the other hand, have a reputation for long life principally because they have no valves in their compression/expansion cycle loop. Instead, they are capillary expansion systems, without any throttling valve and without any cycling bypass valves. In such systems the compressor itself is cycled in order to control cooling.
One method often used in the prior art to reduce cycling failures was to deliberately undersize the refrigeration system for the anticipated load, so that the system would be kept running in the cooling mode most of the time. Another system included three separate compressors all on the same inflow and outflow lines. One, two or three compressors would be operational, depending on the load at any given time.
For very high ambients, conventional vapor-compression refrigeration simply has not been able to operate, particularly without water cooling, as in a refrigerated truck crossing the desert. Some such vehicles have had to use dead loss evaporative refrigeration systems in hot environments. In a dead loss system a cryogen such as liquid nitrogen passes through coils, evaporates and is exhausted to atmosphere as waste.
No refrigeration system of the prior art has resulted in the advantages of the present invention described below, with respect to avoidance of destructive valve cycling and bypassing of flow on a proportional basis in a manner resulting in smooth transitions of flow, continuous compressor operation even at no load, and protection of the system against overwork and overheating as limit levels of pressure drop or temperature are approached.